Replace vim selection with output of shell command

I need to pass some selected text in vim to a curl command as a parameter value. For example. I need to be able to run

curl -sSd css="body { border-radius: 5px; }" http://prefixr.com/api/index.php

from vim. Obviously, the "body { border-radius: 5px; }" part will be dynamic. Usually, a visual mode selection in vim.

How do I get the selected text and pass it as a parameter to curl?